No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 433 



cleavage plane lying between the entomeres as the basis of 

 orientation. In Amphitrite, according to Mead, " if the second 

 cleavage furrow is followed around the whole ^%%, its course is 

 found to be an irregular zigzag, but its general direction is at a 

 considerable angle to the sagittal plane." In both Nereis and 

 Amphitrite the portions of the first two cleavage furrows lying 

 between the ectomeres have essentially the same axial relations. 

 It is only in the portions of these furrows lying between the 

 entomeres that the axial relations vary to any marked degree, 

 and it is probable that in Nereis, as in Crepidula, the transverse 

 direction of the lower portion of the first cleavage furrow is 

 due to the shifting of the entomeres in relation to the median 

 axis. 



It is very probable that if the lower portion of the first cleav- 

 age furrow were not shifted in relation to the upper, the whole 

 cleavage plane would be, in every case, oblique to the future 

 median axis of the embryo. In fact, it may be said that, when- 

 ever the lower portion of the first cleavage fnrroiv has a direction 

 differejit from that of the upper portion, its direction has been 

 altered in relation to a part of this furrow which lies at a 

 remarkably constant angle to the futjire median axis of the em- 

 bryo ; this is consequently tantamojint to becoming shifted in 

 relation to the median axis itself. In such cases the lower por- 

 tion of the first cleavage furrow can no longer be considered 

 indicative of the direction of the first cleavage plane. 



It is apparently, therefore, a universal characteristic of forms 

 with spiral cleavage that the first cleavage plane is oblique to 

 the future longitudinal axis of the embryo. As far as is known, 

 the first cleavage plane, in forms with normal or unreversed 

 cleavage, always makes a positive angle with the future median 

 axis, while it makes a negative angle with this axis in forms 

 whose cleavage is of the reversed or sinistral type. These 

 facts, I believe, are not devoid of significance in relation to the 

 general theory of spiral cleavage. In the previous section the 

 suggestion was made that the difference in the direction of the 

 first spiral cleavages in Crepidula and Planorbis might be due 

 to the circumstance that the first cleavage plane cuts a pre- 

 formed longitudinal axis in one form at a positive, and in the 



